zfsd: Check for error from zpool_vdev_online

Onlining a vdev can fail. Log the error if it does.

Reviewed by: mav, asomers
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D30882

(cherry picked from commit 53b438b2425c374f6147ac80b3330a9ec08432bb)